The organisms studied are the human malaria parasite, Plasmodium falciparum, and the related Apicomplexan parasite Toxoplasma gondii, with emphasis on whole genome approaches to their genetics and molecular and biological phenotypes. Computer analyses, including application of several new algorithms, have been carried out, in close collaboration with experimental laboratories, in response to questions arising about the structures, functions, interactions and genetics of the genes and gene products, including those involved in drug and vaccine development,drug resistance and evasion of the immune response, and genetic diversity and evolution in parasite populations. These analyses include genetic mapping, quantitative and population genetics, and comparative approaches with other extensively sequenced parasites, pathogens and eukaryotic genomes. Several Plasmodium species show extreme bias in the residue compositions of both genes and proteins. These compositional properties, together with questions about the roles of low-complexity, repeat, nonglobular and conformationally mobile parts of proteins, are relevant to aspects of the pathogenicity of these organisms and their interactions with the mammalian hosts. Aspects of malaria parasite biology under study include the evolution and mechanisms of complex drug resistance, antigen structure and variation, evasion of the immune response, novel aspects of genome organization and gene expression.